Wireless communication systems are widely known in which base stations (BSs) communicate with user equipments (UEs) (also called subscriber or mobile stations) within range of the BSs.
The geographical area covered by one or more base stations is generally referred to as a cell, and typically many BSs are provided in appropriate locations so as to form a network covering a wide geographical area more or less seamlessly with adjacent and/or overlapping cells. (In this specification, the terms “system” and “network” are used synonymously). Each BS divides its available bandwidth, i.e. frequency and time resources, into individual resource allocations for the user equipments. There is a constant need to increase the capacity of such systems, and to improve the efficiency of resource utilisation, in order to accommodate more users, more data-intensive services and/or higher data transmission rates.
OFDM (Orthogonal Frequency Division Multiplexing) is one known technique for transmitting data in a wireless communication system. An OFDM-based communications scheme divides data symbols to be transmitted among a large number of subcarriers, hence the term frequency division multiplexing. Data is modulated onto a subcarrier by adjusting its phase, amplitude, or both phase and amplitude. The “orthogonal” part of the name OFDM refers to the fact that the spacings of the subcarriers in the frequency domain are specially chosen so as to be orthogonal, in a mathematical sense, to the other subcarriers. In other words, they are arranged along the frequency axis such that the sidebands of adjacent subcarriers are allowed to overlap but can still be received without inter-subcarrier interference. In mathematical terms, the sinusoidal waveforms of each subcarrier are called eigenfunctions of a linear channel, with the peak of each sinusoid coinciding with a null of every other sinusoid. This can be achieved by making the subcarrier spacing a multiple of the reciprocal of the symbol period.
When individual subcarriers or sets of subcarriers are assigned to different user equipments, the result is a multi-access system referred to as OFDMA (Orthogonal Frequency Division Multiple Access). The term OFDM as used in the art is often intended to include OFDMA. The two terms may therefore be considered interchangeable for the purposes of the present explanation. By assigning distinct frequency/time resources to each user equipment in a cell, OFDMA can substantially avoid interference among the user equipments within a given cell.
In a wireless communication system such as LTE, data for transmission on the downlink is organised in OFDMA frames each divided into a number of subframes. Various frame types are possible and differ between FDD and TDD for example.
Meanwhile, on the uplink, in view of the relatively unfavourable PAPR (peak-to-average-power ratio) properties of ODMA, an alternative scheme called SC-FDMA (Single Carrier Frequency Division Multiple Access) is used in LTE, which allows a better balance between uplink range and UE amplifier cost. In an SC-FDMA signal, each subcarrier used for transmission contains information of all the transmitted symbols whereas individual subcarriers of an OFDMA signal carry only information on specific symbols.
A technique called MIMO, where MIMO stands for multiple-input multiple-output, has been adopted in several commercial wireless systems including LTE due to its spectral efficiency gain, spatial diversity gain and antenna gain. This type of scheme employs multiple antennas at the transmitter and/or at the receiver (often at both) to enhance the data capacity achievable between the transmitter and the receiver. Typically, this is used to achieve an enhanced data capacity between one or more BSs and the UEs served by the BSs).
By way of example, a 2×2 MIMO configuration contains two antennas at the transmitter and two antennas at the receiver. Likewise, a 4×4 MIMO configuration contains four antennas at the transmitter and four antennas at the receiver. There is no need for the transmitter and receiver to employ the same number of antennas. Typically, a BS in a wireless communication system will be equipped with many more antennas in comparison with a UE (such as, for example, a mobile handset), owing to differences in power, cost and size limitations.
The term channel is used to describe the frequency (or equivalently time delay) response of the radio link between a transmitter and a receiver. The so-called MIMO channel (or “channel”) contains all the subcarriers (see the discussion on subcarriers above), and covers the whole bandwidth of transmission. A MIMO channel contains many individual radio links. The number of these individual radio links, which may be individually referred to as single-input single-output (SISO) channels (also called subchannels), is Nr×NT, where NT is the number of antennas at the transmitter and Nr is the number of antennas at the receiver. For example, a 3×2 MIMO arrangement contains 6 links, hence it has 6 SISO channels.
Considering the simplified 2×3 MIMO system schematically represented in FIG. 1, it can be seen that antenna R0 of receiver R receives transmissions from each of the transmitter antennas T0, T1 and T2 of transmitter T. Similarly, receiver antenna R1 receives transmissions from transmitter antennas T0, T1 and T2. Therefore, the signal received at the receiver comprises (or is made up of) a combination of the transmissions (i.e. of the SISO channels) from the transmitter antennas. In general, SISO channels can be combined in various ways to transmit one or more data streams to the receiver.
The number of simultaneously transmitted streams the MIMO channel can support is commonly referred to as the “channel rank”, and the number of streams actually transmitted is referred to as “transmission rank”. The transmission rank typically needs to be adapted to suit the current channel characteristics and hence avoid excessive inter-stream interference. A more general definition of transmission rank is the number of complex-valued independent modulation symbols transmitted per time-frequency resource.
FIG. 2 is a conceptual diagram of a more generalized MIMO system. In FIG. 2, a transmitter transmits signals utilizing NT transmitting antennas, and a receiver receives the signals from the transmitter utilizing Nr receiving antennas. In order to create a mathematical model of the characteristics of the overall MIMO channel, it is necessary to represent the individual SISO channels between the transmitter and receiver. As shown in FIG. 2, the individual SISO channels are represented by H0,0 to HNr−1,NT−1, and as suggested in the Figure, these form terms of a matrix commonly called the channel matrix or channel response matrix H. It will be recognised that H0,0 represents the channel characteristics (for example, channel frequency response) for transmitting signals from the transmitting antenna 0 to the receiving antenna 0. Similarly, “HNr−1, NT−1” represents the channel characteristics for transmitting signals from the transmitting antenna NT−1 to the receiving antenna Nr−1, and so on.
In FIG. 2, the symbols x0 to xNT−1, which represent the signal elements transmitted using the transmitting antennas 0 to NT−1, together form a transmitted signal vector x (i.e. x=(x0, x1, x2, . . . , xNT−1)T), where ( )T indicates the vector transpose. Likewise, the received signals elements y0 to yNr−1 received by receiving antennas 0 to Nr−1 together form a received signal vector y (i.e. y=(y0, y1, y2, . . . yNr−1)T). The relationship between the vectors y and x for the simplified system shown in FIG. 2 (and also that shown in FIG. 3) may be modeled by the basic MIMO system equation:y=Hx+n  (I)where H is the channel matrix described above and n is a vector representing noise. Noise elements n0 to nNr−1 are illustrated in FIG. 2 and represent noise in the respective received signal elements y0 to yNr−1. Hence, the noise vector n is given by n=(n0, n1, n2, . . . , nNr−1)T.
It should be noted that, despite the name “multiple-input multiple-output”, MIMO systems can operate even if one of the transmitter and the receiver has only one antenna (i.e. even if NT=1 or Nr=1).
MIMO transmission schemes may be described as “non-adaptive” and “adaptive”. In the non-adaptive case, the transmitter does not have any knowledge of the channel properties and this limits performance, as it cannot take account of changes in conditions which cause changes in the state of the channel. Adaptive schemes rely on channel knowledge which may be obtained, for example, by the feedback of information (channel-state information or CSI) from the receiver to the transmitter, allowing modification of the transmitted signal to account for changing conditions and to maximise data throughput. Adaptive MIMO schemes may be further described as “closed-loop” (i.e. operating with the benefit of channel state feedback) or “open-loop” (i.e. without channel state feedback). A combination is possible in the sense that the scheme may be “closed-loop” with respect to some aspects (e.g. feedback of received power”) and “open-loop” with respect to other aspects (e.g. no feedback related to the channel matrix). The present invention is concerned primarily with closed-loop MIMO schemes.
The feedback just described is important, in particular, in so called FDD (Frequency Division Duplex) systems, where uplink transmissions (i.e. transmissions from user equipment to base station) and downlink transmissions (vice-versa) employ two different carrier frequencies. Because of the frequency change, the uplink and downlink channels are different and CSI needs to be fed back in order to provide an adaptive scheme; in particular so that the transmitter can perform so-called “link adaptation” in order to account for channel variations (such as changes in the channel state) when transmitting signals. On the other hand, in so-called TDD (Time Division Duplex) systems, the uplink and downlink are transmitted in two adjacent time slots on the same frequency. The two time slots are generally within the channel coherence time, meaning that it can be reasonably assumed (e.g. with the same antennas used in the uplink and downlink directions) that the channel state does not change, so information relating to the channel matrix need not be fed back. The transmitter can estimate the channel from the received signal on the reverse link, usually aided by the insertion of pilots or known waveforms by the transmitter into the signal sent on the reverse link. This is often referred to as “uplink sounding”. However, it may not always be desirable to incur the overhead of uplink sounding, in which case closed-loop techniques offer an alternative.
FIG. 3 is a diagram representing a MIMO system similar to that shown in FIG. 1, but more generalised. MIMO system 1 comprises a transmitter 2 which comprises a plurality of transmitting antennas (0), (1), . . . , (NT−1) and a receiver 3 which comprises a plurality of receiving antennas (0), (1), . . . , (Nr−1). The transmitter 2 transmits symbols 0, 1, . . . , NT−1 using the NT transmitting antennas. The symbols can be created from one data stream, referred to as vertical encoding, or different data streams, referred to as horizontal encoding. In addition, each transmitted symbol corresponds to, for example, one-bit data if the modulation method is binary phase-shift keying (BPSK), or corresponds to two-bit data if the modulation method is quadrature phase-shift keying (QPSK). These concepts will be familiar to those skilled in the art. The receiver 3 receives the signals transmitted from the transmitter 2 using the Nr receiving antennas, and it comprises a signal regeneration unit 4 which regenerates the transmitted symbols from the signals received.
As indicated by the arrows in FIG. 3, the signals transmitted from a plurality of the transmitting antennas are received by a plurality of receiving antennas, giving rise to Nr×NT possible subchannels in total. In other words, the signals transmitted from the transmitting antenna (0) are received by receiving antennas (0) through (Nr−1), the signals transmitted from the transmitting antenna (1) are received by receiving antennas (0) through (Nr−1), etc. The characteristics of the subchannel which propagates the signals from the i-th transmitting antenna to the j-th receiving antenna are expressed as “Hji” and form one component term of the Nr×NT channel matrix H.
The maximum number of independent data streams that can be usefully transmitted in parallel over the MIMO channel is given by the lower of NT and Nr and is further limited by the rank of the matrix H. The transmission quality depends on the matrix H and, for example, degrades significantly in case the singular values of the matrix are not sufficiently strong, such as where antennas are not sufficiently de-correlated, for example in an environment with little scattering or when antennas are physically close together.
In LTE, up to 2 code words can be mapped onto different so-called layers. The number of layers for transmission is typically chosen to be less than or equal to the rank of the matrix H, and there is a fixed mapping between code words to layers. Precoding on the transmitter side can be achieved by applying a precoding matrix W to the signal before transmission. The optimum available precoding matrix W is selected from a predefined “codebook”, which is known at both the base station(s) and UE side. The UE selects the optimum available precoding matrix (the one offering the highest data rate) based on its knowledge of the channel, and indicates its preferred precoding matrix to the transmitter side, via a precoding matrix index (PMI) for example. PMI is one kind of channel state information (CSI) mentioned earlier. Note that in LTE, while the precoder used at the BS is likely to be designed on the basis of the UE feedback, this precoder is not necessarily restricted to be one of codebook entries.
By way of further background explanation, a MIMO-OFDM transmitter and a MIMO-OFDM receiver will be briefly outlined with reference to FIGS. 4 and 5 respectively. In the OFDM transmitter schematically shown in FIG. 4, high-speed binary data is encoded (convolutional code is an example), interleaved, and modulated (using a modulation scheme such as BPSK, QPSK, 64QAM, and the like). Independent channel encoders may be used for each transmitting antenna. Subsequently, the data is converted into parallel low-speed modulated data streams which are fed to M subcarriers. The output from each encoder is carried separately on a plurality of subcarriers. The modulated signals are frequency-division multiplexed by M-point Inverse Fast Fourier Transform (IFFT) and the guard interval is added. The resulting OFDM signal is converted into an analog signal by a D/A converter and is upconverted into RF band and transmitted over the air.
At the MIMO-OFDM receiver schematically shown in FIG. 5, the received signals from the Nr receiver antennas are filtered by a band pass filter (BPF), and then down-converted to a lower frequency. The down-converted signal is sampled by ND converter (namely, converted into a digital signal), and the guard interval is removed before the sampled data is fed to the M-point Fast Fourier Transformer (FFT). After Fourier transformation is performed on each of the signals received through the Nr receiver antennas, they are fed to the MIMO signal processing unit 11. The MIMO signal processing unit 11 comprises the signal regeneration unit 4 (as shown in FIG. 3) which performs processing to compensate for the channel characteristics.
It should be noted that, for the purposes of explanation, the above discussion focused mainly on the case of a single transmitter sending MIMO signals to a single receiver or in other words to a set of antennas in one location (so-called Single User or SU-MIMO), but of course practical MIMO wireless communication systems are generally much more elaborate than this, providing many mutually adjacent cells in which base stations transmit over respective MIMO channels to one or more UEs simultaneously. In fact, the present invention is largely directed at these more elaborate systems, and issues associated with them, as discussed below. The term Multi-User MIMO or MU-MIMO refers to techniques which rely on precoding to exploit the geographical separation of users' respective antennas, allowing signals to be transmitted to and received from a plurality of user equipments in the same frequency band simultaneously.
As explained above, the means by which frequency resources are utilised in conventional MIMO schemes prevents or significantly limits interference among user equipments within a given cell. In other words, intra-cell interference is substantially avoided. However, in the more elaborate multi-cellular networks discussed in the previous paragraph, the benefits of MIMO transmission can often be limited by inter-cell interference.
Inter-cell interference may arise, for example, because the frequency resources (i.e. the carriers and subcarriers) utilised by base stations in transmitting data to user equipments in one cell are identical to the frequency resources utilised by base stations in transmitting data to user equipments in an adjacent cell. In other words, in the kinds of wireless communication systems in which the present invention may find use, there is likely to be, using terminology common in the art, 1:1 frequency reuse between adjacent cells. The effect of this can be particularly significant for so-called “cell-edge users” located near the boundary between cells. For a cell-edge user, the distance to the one base station currently serving that user equipment may be roughly the same as, or only marginally different to, the distances to the base stations that are in adjacent cells. It should also be noted that the received signal strength is typically highly correlated with distance. As a result, from the point of view of the user equipment near the cell edge, the signal strength received from the serving base station may be only marginally stronger than, or approximately the same as, the signal strength from the base stations in the adjacent cells, as seen by the cell-edge user. And because common frequency resources may be used in adjacent cells (i.e. there is simultaneous use of substantially identical transmission frequencies in adjacent cells), signals being transmitted in the adjacent cells can often interfere with data being transmitted to the cell-edge user.
One method which has been proposed for addressing this difficulty is to coordinate the MIMO transmissions among multiple base stations (i.e. coordinating transmissions in adjacent or nearby cells) to eliminate or reduce this inter-cell interference. A full explanation of the techniques employed to achieve this coordination is not necessary for the purposes of this explanation. For present purposes it is sufficient to note that this coordination can reduce or eliminate inter-cell interference among coordinated cells (or coordinated portions of cells) and this can result in a significant improvement in the coverage of high data rates, cell-edge throughput and/or overall system throughput. However, the trade-off for this improvement is that the coordination of transmissions in multi-cellular MIMO systems requires channel state information (CSI) and data information to be shared among the coordinated base stations. This in turn results in a significant additional burden on the system's transmission and data capacity resources. In particular, for FDD systems, base station channel knowledge is mainly obtained by user equipment (UE) feedback (UE feedback is also useful in TDD-based systems). Since multiple cells or sectors participate in the coordinated transmission, the amount of channel knowledge required to be fed back increases linearly with the number of cooperating cells (or the number of cooperating cell sectors). In other words the UE may need to feed back information on each cell participating in the coordinated transmission, to the base stations providing those cells. It will be appreciated that this can place a heavy burden on the uplink channel particularly.
As explained in the previous paragraph, coordinated multi-cell MIMO transmission/reception (also often referred to as coordinated multi-point transmission/reception or CoMP) may be used to improve the coverage of high data rates, cell-edge throughput and/or to increase system throughput. The downlink schemes used in CoMP may be considered to fall into the following two categories:                “Coordinated Scheduling and/or Coordinated Beamforming (CS/CB)” and        “Joint Processing/Joint Transmission (JP/JT)”.        
An additional technique which may be employed is aggregation of multiple carriers (CA) to increase the available peak data rate and allow more complete utilisation of available spectrum allocations.
Incidentally, those skilled in the art will be generally familiar with the basics and underlying principles of beamforming, which is a signal processing technique that makes use of constructive and destructive interference to assist with directional signal transmission and/or reception. Further explanation of beamforming is therefore not required here.
In CS/CB, data to a single UE is transmitted from one transmission point, but decisions regarding user scheduling (i.e. the scheduling of timings for transmissions to respective user equipments) and/or beamforming decisions are made with coordination among the cooperating cells (or cell sectors). In other words, scheduling/beamforming decisions are made with coordination between the cells (or cell sectors) participating in the coordinated scheme so as to prevent, as far as possible, a single UE from receiving signals from more than one transmission point.
On the other hand, in JP/JT, data to a single UE is simultaneously transmitted from multiple transmission points to (coherently or non-coherently) improve the received signal quality and/or cancel interference for other UEs. In other words the UE actively communicates in multiple cells and with more than one transmission point at the same time.
Further details of CoMP as applied to LTE can be found in the document:
3GPP TR 36.814: “Further advancements for E-UTRA physical layer aspects (Release 9)”, V1.0.0, 26.02.200926
In CA, discrete frequency bands are used at the same time (aggregated) to serve the same user equipment, allowing services with high bandwidth demands (up to 100 MHz) to be provided. CA is a feature of LTE-A (LTE-Advanced) which allows LTE-A-capable terminals to access several frequency bands simultaneously whilst retaining compatibility with the existing LTE terminals and physical layer. CA may be considered as a complement to JP for achieving coordination among multiple cells, the difference being (loosely speaking) that CA requires coordination in the frequency domain and JP in the time domain.
FIG. 6 schematically illustrates the working principles of the two above-mentioned categories of downlink transmission used in CoMP, although it should be noted that the way the base stations are illustrated relative to the distribution of the cells in FIG. 6 may not reflect the true distribution of base stations vis-à-vis cells in a practical wireless communication system. In particular, in a practical wireless communication system, the cells extend further than the hexagons shown in the Figure so as to overlap to some extent, allowing a UE to be within range of more than one base station at the same time. Furthermore, it is possible, in LTE for example, for the same base station (eNodeB) to provide multiple overlapping cells. Nevertheless, FIG. 6 is sufficient for present purposes to illustrate the principles of CS/CB and JP downlink transmission schemes respectively, used in CoMP.
Joint Processing (JP) is represented in FIG. 6(a) in which cells A, B and C actively transmit to the UE, while cell D is not transmitting during the transmission interval used by cells A, B and C.
Coordinated scheduling and/or coordinated beamforming (CS/CB) is represented in FIG. 6(b) where only cell B actively transmits data to the UE, while the user scheduling/beamforming decisions are made with coordination among cells A, B, C and D so that the co-channel inter-cell interference among the cooperating cells can be reduced or eliminated.
In the operation of CoMP, UEs feed back channel state information. The channel state information is often detailed, and often includes measurements of one or more of channel state/statistical information, narrow band Signal to Interference plus Noise Ratio (SINR), etc. The channel state information may also include measurements relating to channel spatial structure and other channel-related parameters including the UE's preferred transmission rank and precoding matrix.
As explained above, feedback of channel state information allows modification of the transmitted signal (typically modification by the base station(s) prior to transmission) to account for changing channel conditions and to maximise data throughput. More specifically, it is often done in order to perform precoder design, link adaptation and scheduling at the base stations. As also explained above, for FDD systems, to achieve equivalent detail of channel knowledge for each cell, the total amount of channel information needed to be fed back increases linearly with the number of cooperating cells (or sectors of cells), and this creates a heavy additional burden for the uplink channel particularly.
Conventionally, CSI reporting is provided for without taking into account multiple cells or their relative significance in communications with a specific UE. However, as explained above, in CoMP there is coordination between cells, and in fact, in the case of the joint processing (JP) downlink transmission scheme discussed above, data to a single UE is simultaneously transmitted from multiple transmission points. Note that “coordination between cells” could be understood to include cells on different carrier frequencies (i.e., CA), which may be supported by one base station or a number of co-located base stations, as well as cells supported by geographically-separated base stations (i.e., COMP).
Therefore, it its worth Investigating feedback schemes that can more appropriately deal with a UE's communications with multiple cells, and which can save the feedback overhead for the uplink channel used for multi-cell DL MIMO transmissions.